Spacing in coaxial tubes system

ABSTRACT

Coaxial tubes are thermally insulated from each other by means of a spacer comprised of loosely piled tapes which have been sewn together, and helically wound on the respective inner tube.

United States Patent Ziemek et a1. 1 51 June 20, 1972 [54] SPACING IN COAXIAL TUBES SYSTEM [72] Inventors: Gerhard Ziemek, l-lannover, Friedrich [56] References cmd Schatz, Langenhagen, both of Germany UNITED STATES PATENTS 1 Assigneer Kflbel-und Metallwerke Gutehoff- 3,592,238 7/1971 Scheffler et a1 138/1 14 nungshu Akflengesellschaft, Hannover, 3,563,198 2/1971 Johnston ..2/81 X Germany 1,691,793 11/1928 Mottweiler e! a] 138/147 [221] Filed: Sept. 21, 1970 3,332,446 7/1967 Mann ..138/148 X [21] App]. No.: 73,688 Primary Examiner-Wi11iam E. Wayner Attomey-Smyth, Roston & Pavitt and Ralf H. Siegemund [30] Foreign Application Priority Data [57] ABSTRACT Oct. 14, 1969 Germany up 19 51 Coaxial tubcs are thermally insulated from each other y means of a spacer comprised of loosely piled tapes which have [52] 11.8. CI "138/114, 138/148, 112/440 been sewn together, d helically wound on the respective [51] ..F16l11/12,F16159/12 innertuba [58] Field ot'Search ..174/29;138/112,137,147,

6 Claims, 3 Drawing Figures generally loosely positioned SPACING IN COAXIAL TUBES SYSTEM The present invention relates to a method of thermally insulating an inner tube disposed in an outer tube of larger diameter.

In copending application of common assignee, Ser. No. 852,810, of Aug. 25, 1969 now US. Pat. No. 3,592,238, a pipe or tube system is disclosed in which telescoped tubes are maintained in spaced apart, coaxial relationship by means of a particular spacer. The tube system as disclosed in that application is to be used in an environment and under circumstances resulting in a steep temperature gradient between respective two concentrical tubes. In particular such a system is used in and as a part of cryogenic cable system or as a conduit for fluids that are significantly hotter or colder than the environment. The particular spacer disclosed in the copending application is comprised of loosely stacked or piled tapes of thermally insulating material. The tapes of the stack or pile are fastened to each other in isolated and individual, spaced apart points. The stack of loosely piled tapes has been wound helically upon the inner one of a pair of concentrically disposed tubes thereby defining and establishing the position of the respective outer ones of these tubes.

The objective accomplished was that adjacent ones of the tapes of such pile (formed into a helix) engage each other in surface to surface contact in incremental areas only and along thin and narrow lines, so that the total area of intimate surface to surface contact as between two adjacent tapes is very small as compared with areal size of each tape.

As a consequence, the pile of tapes provide conduction for thermal energy in direction of stacking at a very small cross section as far as heat conduction through solid material is concerned. The points of contact as between respective two adjacent tapes are statistically distributed throughout the pile or stack so that the average length for the heat propagation path through the pile is significantly larger than the thickness of the pile. Additionally, there is a temperature jump at each of the contact points because of an inherent gap that is present even in a microscopically surface to surface contacting" area. From every point of view, the resulting helix has very low thermal conductivity, and little thermally energy flows between the coaxially disposed tubes having this helix as spacer.

The present invention now relates to improvement in the making of an arrangement of this type. In accordance with the invention, tapes are loosely piled in a stack, and a thread is run through the stack or pile, for fastening them together. Thus, the tapes of the pile are actually sewn together. The thread itself should also have low thermal conductivity. If the sewing needle has diameter larger than the thread, the resulting perforations in each tape are larger accordingly. As a consequence, the individual tapes may well slide to some extent relative to each other. This has the advantage, that the spacer constructed from these piled and sewn tapes can be wound onto the one tube without twisting.

Now, in practicing the invention, the tapes may actually be sewn together with a conventional sewing machine, with a stitch length selected in a range from below an inch up to 8 inches or thereabouts. In toto, the inventive method can be practiced as follows. First, an inner tube is formed. Next a plurality of tapes are stacked in a pile, sewn together and the stack or pile is wound onto the tube. Finally, a second tube may be formed, for example, by wrapping a strip around the tube-tape-pile assembly to establish a tubular envelope, firmly seated on the spacer.

Conceivably, the tapes when cold are somewhat brittle, so that it is of advantage to heat them prior to or during sewing. As stated, the needle used for running the thread through the tapes should have a diameter that is larger than the diameter of the thread. This offers the added advantage of reducing friction during sewing; also, the thread should be very smooth for low surface friction. This way, a relatively large pile of tapes can be sewn together. Preferably, a plastic thread is used, consisting of the same material as the tape.

While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention, it is believed that the invention, the objects and features of the invention and further objects, features and advantages thereof will be better understood from the following description taken in connection with the accompanying drawings in which:

FIG. 1 illustrates a perspective view into a tube system established in accordance with the preferred embodiment of the present invention;

FIG. 2 illustrates a perspective view through a pile of tapes as sewn together; and

FIG. 3 illustrates a enlarged section view of a detail of the pile of tapes.

The tube system illustrated representatively in FIG. 1 illustrates three coaxially arranged tubes, there being an inner tube 1, an intermediate tube 2 and an outer tube 3. It is assumed that inner tube 1 receives, for example, an electrical cable to be operated in the super conductive state. Altematively, tube 1 may be used for conduction of very cold or very hot fluids.

In case the tube assembly refers to a cryogenic cable, tube 1 may be filled with liquid helium having a temperature of 4 Kelvin to maintain the cable in tube 1 in the super-conductive state. The space between tubes 2 and 3 may be filled with liquid nitrogen having a temperature of approximately 77 Kelvin. Thus, tubes 1 and 2 may have a temperature difference of somewhat above 70 Centigrade. Under such circumstances good thermal insulation is mandatory for successful operation.

The space between tubes 1 and 2 is evacuated so that there is practically no conduction of thermal energy by convection or any other molecular heat transfer such as conduction through gas etc. For example, the pressure in the ring space has been reduced to a value so that the mean free path becomes comparable with the radial gap width of the space between tubes 1 and 2.

This being the background, a particular spacer 4 is provided to maintain tubes 1 and 2 in concentric position. A similar spacer may be provided between tubes 2 and 3. The spacer is made in accordance with the principle feature of the present invention as is now described with particular reference to FIGS. 2 and 3.

The spacer is constructed by forming a stack or pile from a plurality of individual tapes 5. Actually, the tapes have been stacked rather neatly, but not pressed into intimate contact so that the pile remains rather loose. A piece of thread 9 is run through the tapes for fastening them to each other. Points 6, 7 and 8 show where the several tapes are interconnected. These connecting points are about 2 to 20 cm apart. The tapes are shown in their thickness dimension in a grossly exaggerated scale, particularly as compared with their width dimension; in reality, the tapes are thinner than the thread. Also, the number of tapes stacked is considerably larger than shown.

The thread is run through the tapes by means of conventional sewing equipment or the like. For producing the spacer in accordance with the invention, it is of particular advantage, if a threading or sewing needle is used, having-a diameter larger than the diameter of the thread. As can be seen particularly from FIG. 3, the individual tapes are perforated by the needle to obtain perforations larger than the thickness of the traversing thread. From this, a number of considerable advantages result.

The thread actually runs through a plurality of more or less misaligned apertures, as even after sewing, the individual tapes may slide relative to each other for a distance, which is given by the difference between the diameter of an aperture or perforation, and diameter of the thread. As the sewn pile is wound upon the inner tube 1, the entire pile exhibits a particular flexibility due to the fact that the tapes are permitted may slide relative to each other, though over a limited range. This way, internal tension in the stack, tearing etc., is prevented. Also, the fastening of the tapes in the pile is not loosened.

The tapes as well as the thread may well be made of the same material, for example polyamide, e.g., as traded under the conventional designation nylon. This material has been found to be quite suitable, particularly because the coeflicient of surface friction for this material is very low. Thus, the thread is threaded through the many pierced tapes, at comparative ease. Also, it was found, that this material has very low thermal conductivity.

The individual tapes have smooth or rippled surface. In either case, adjacent tapes do not engage each other in broad surface to surface contact but in isolated areas, regions, lines, etc. only, so that the total area of immediate contact and engagement is small as compared with the surface dimension of each of the tapes. The sewn pile of tapes as an entity has very low thermal conductivity, considerably lower than the bulk conductivity of the material(s) used for tapes and thread.

Another advantage of using relatively large perforations is to be seen in a very limited region of surface to surface contact as between tape and thread. The thread will generally engage a tape edgewise in a perforation. Moreover, as the tapes slide relative to each other, vertically aligned tapes will engage the thread in different portions of the misaligned perforations so that there is no direct straight through transfer path for thermal energy through the stack of tapes adjacent the thread. The thread itself is a straight through transfer path, but the thread itself is thin and by selecting the stitch length as large as possible, the total cross-sectional area for heat transfer lengthwise through the thread is still very low, so that the average path length for heat conduction through the pile is hardly increased. Also, the pile should be sewn rather loosely to avoid forcing of the tapes into intimate contact over large surface areas, particularly around the apertures.

The invention is not limited to the embodiments described above but all changes and modifications thereof not constituting departures from the spirit and scope of the invention are intended to be included.

We claim: 1. Method of thermally insulating a first tube which is disposed in a second tube of larger diameter, comprising the steps of piling a plurality of tapes having low thennal conductivity to form a loosely piled stack of tapes;

fastening the pates to each other by running a flexible thread through perforations of the tapes, the perforations being larger than the thread so that adjacent tapes may shift relative to each other to a limited extent; and

helically disposing the stack of tapes on the first tube as spacer for the second tube.

2. Method as in claim 1, the fastening step consisting of sewing the tapes together.

3. Method as in claim 1 the fastening step carried by use of a needle having diameter larger than the diameter of the thread. 4. Method as in claim 1 using thread of a plastic material.

5. Method as in claim 1, including the step of heating the piled tapes during the fastening step.

6. Method as in claim 3, the fastening step carried out by running a single thread up and down and completely through the pile for each stitch in a meandering pattern therein. 

1. Method of thermally insulating a first tube which is disposed in a second tube of larger diameter, comprising the steps of piling a plurality of tapes having low thermal conductivity to form a loosely piled stack of tapes; fastening the pates to each other by running a flexible thread through perforations of the tapes, the perforations being larger than the thread so that adjacent tapes may shift relative to each other to a limited extent; and helically disposing the stack of tapes on the first tube as spaceR for the second tube.
 2. Method as in claim 1, the fastening step consisting of sewing the tapes together.
 3. Method as in claim 1 the fastening step carried by use of a needle having diameter larger than the diameter of the thread.
 4. Method as in claim 1, using thread of a plastic material.
 5. Method as in claim 1, including the step of heating the piled tapes during the fastening step.
 6. Method as in claim 3, the fastening step carried out by running a single thread up and down and completely through the pile for each stitch in a meandering pattern therein. 